Is there any innocent scenario in which a pilot would choose to turn transponder off?
And if not, why can they be turned off at all?
If you know, thanks for explaining.

Usually turned off on the ground else everyone's units confuse radar and clog things up. Also, like anything electronic, transponders can go nutso and would need to be reset or shut down. Some pilots like to turn them off when changing codes (this more for older models).

Thanks for a clear explanation - much more than I got from CNN!
Appreciate your time-

When a pilot goes out to take off, he'll be instructed by the tower to put a series of numbers in the transponder, and they may be asked to squawk--or push a little button on the transponder. The transponder tells them who's who on their radar screens in the immediate area.

After the pilot leaves the tower's controlled air space, he'll switch the transponder to "1200" which indicates that the plane is flying under Visual Flight Rules.

Bamaman, that's generally correct, for some general aviation aircraft, but doesn't accurately apply for airline operations but corrections are trivial and won't add to the discussion.

VonVan, another reason: possibility of fire or other electrical problem where being able to turn off various pieces of equipment (or the entire electrical bus) makes it possible to contain or eliminate the problem. If there were a smouldering short circuit in the transponder, for example, cutting power is smart.

You'll find that virtually every aircraft system can be turned off by the pilots, when you are 5 miles up it's nice to know that if smoke starts coming out of a panel you can do something about it

I'm sure that's true, but if you're going down or being hijacked, it might be nice for someone to know where you are.

Indeed, but things catching fire are more common than hijacking - and I'm pretty sure that a pilot would rather put out a fire than be happy in the knowledge that a crash is fully reported

You know what - I think you're right.